Effects of Additives and Impurity on the Adhesive Behavior of the NiAl(110)/Al 2 O 3 (0001) Interface: An Ab Initio Stud
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THERMAL barrier coatings (TBCs) are used in high-performance gas turbine engines to protect turbine blade components from elevated temperatures and to improve the life and maximize the fuel efficiency of the engine.[1] Typically, TBC consists of a thermally insulating ceramic-based top coat deposited on a metallic bond coat. The bond coat acts as a bridge connecting the substrate alloy to the top coat and minimizes the strains generated due to thermal expansion mismatch between the top coat and the substrate.[2] During prolonged high-temperature exposures, a thermally grown oxide (TGO) Al2O3 layer that provides oxidation and corrosion resistance forms along the bond coat and top coat interface through oxidation of the bond coat. Due to thermal expansion mismatch between TBC and excessive growth of the TGO, microvoids and cracks nucleate, and in turn cause the TGO spall off the bond coat. The durability and lifetime of TBC are largely affected by the stability and the adherence of the TGO/bond coat interface. Therefore, there is a pressing ISIL OZFIDAN, Graduate Student, is with the Department of Physics, University of Ottawa, Ottawa, ON K1N 6N5, Canada. KUIYING CHEN, Professor, is with the Structures & Materials Performance Laboratory, Institute for Aerospace Research, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. Contact e-mail: [email protected] MING FU, Staff Member, is with General Electric, Aviation, Cincinnati, OH 45215-1915. Manuscript submitted February 14, 2011. Article published online September 15, 2011 4126—VOLUME 42A, DECEMBER 2011
need to better understand the interfacial adhesive mechanisms and to find potential approaches to improve the adhesive strength of the interface.[3] Currently, the most used ceramic top coat in gas turbine engine applications is the yttria-stabilized zirconia, owing to its high thermal expansion coefficient and low thermal conductivity.[4] Also, nickel aluminide intermetallic b-NiAl is widely used as the bond coat since it has a high melting temperature and the ability to form a protective and slow growing corundum Al2O3 during the service period.[5] It was reported that impurity sulfur shows a strong tendency to segregate to both the free metal surface and the TGO/bond coat interface, promoting void formation, reducing the interface adherence, and thus causing early spallation.[6–9] Addition of Pt and reactive elements (REs) of Hf, Y, and Zr to the interface was found to alleviate the detrimental effect of S to some extent.[6–15] Pt was reported to add to the bond coat to improve the interface adhesion, mitigate the detrimental effect of S through enhancing the interface adhesion strength, and promote Al diffusion to the interface and reduce void formation.[16–23] The addition of Hf, Zr, and Y into the bond coat was found to lower the weight gain of oxides, inhibit the void formation, enhance the metal-oxide adhesion, and reduce the S diffusion rate to the interface by forming sulfides[7,12–15,24–29] in the bond coat. Cr, however, was shown to accelerate
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